If you are going to get into the world of multi-rotors and you have no idea where to start here I will explain all of the basic components needed for flight. I will not go into tremendous detail because there are many resources for such knowledge available online. I will, however, give you a good idea of where to start and what you will need.
The first things we will cover are the components which make up the multi-rotor itself. They come in many shapes and sizes and have varying levels of complexity but the basic parts will always be the same just in varying quantities. Below are a few examples of multi-rotor flight configurations.
These examples are from the mission planner software which configures the flight controller I used for my project, the APM 2.5. As you can see there are many different configurations for a multi-rotor platform, each with its own advantages and drawbacks. I will be focusing on the X configuration for this project also known as a quad copter. It is a good middle of the road configuration mixing lifting ability with decent stability and agility. Below is a functional diagram of how the different components of a quad copter work together to achieve flight.
At this point I can begin to describe each one of these components in detail.
I start with the battery because it is probably the most important part of the multi-rotor in more than one way. This is because multi-rotor aircraft require a tremendous amount of electricity to operate. Every time you leave the ground all of your motors are working very hard to defy gravity and because they are not airplanes they cannot glide nor can they rely on aerodynamic forces to keep them aloft. Multi-rotors apply pure brute force to keep themselves flying and as such, require large amperages to keep flying. Until very recently there was no battery technology on the market which could provide enough amps for a significant enough amount of time for this type of machine to be viable. Another problem was that even if you had a battery that could provide you with the amount of amps you needed, the battery would inevitably be too heavy and thus weight prohibitive.
Then came the commercialization of Lithium Polymer batteries (LiPO). These batteries have very good power density and are light weight while also providing tremendous amperage for twenty minutes or more depending on the application. Because of these advances small multi-rotor aircraft which could carry payloads for a significant amount of time became possible. The industry has progressed very quickly since then due in large part to China’s rise as an industrial power house.
Most LiPO batteries used in multi-rotor applications come as cells configured in parallel giving you different voltages. One LiPO cell has a voltage range between 2.7 to 4.23 volts. Below you can see the cells from a 3 cell LiPO.
Care must be exercised in charging and maintaining these batteries as they should not be over charged or discharged. Over discharging will lead to a change in the cells’ chemistry which creates gas that will “puff” your battery (as seen in the photo above) as well as ending the cells’ active life prematurely. Overcharging can lead to excessive heat , fire, and gas discharge. Because these batteries can be so volatile, one must always use special balance chargers that can charge and discharge each cell individually to ensure safe operation. Such as the one pictured below:
For the purposes of this project we will be using mainly three and four cell batteries.
The ESC is an electronic speed control unit. It sits between the battery which uses DC current and the brushless motors which run off of multiphase AC current. You can see an example below:
The red and black wires on the left side of the previous image are for the DC current in. The three black wires on the right side of the ESC are for AC current out. The multicolored wire with plug on the left side is used by your Flight Control Board to vary the speed of the motor and also provides a five volt rail which can in turn power the Flight Control Board. That feature is included in ESC units which also have a Battery Eliminator Circuit or BEC built in.
Each ESC unit comes rated to handle a range of amps passing through them. The above example for instance is rated to thirty amps. This means that if the motor/propeller/load combination you are using draws more than thirty amps of current for any amount of time you run the risk of burning out your ESC which in turn would result in a catastrophic failure. This would would be an especially horrible scenario if you were flying your multi-rotor at the time.
There are two types of brushless motors used for remote control applications, brushless outrunners and inrunners. The term outrunner comes from the fact that the motor’s bell housing and the shaft rotate together. The interior of the bell housing has evenly spaced rare earth magnets attached. The center part of the motor, which includes the motor mount, has arms that are wrapped in copper wire wound in a specific manner. When the copper wires are energized by the ESC they act as electromagnets which switch on and off rapidly, interacting with the magnets creating motion. They come in many shapes and sizes.
The wider the bell, the more torque the motor will have. In the case of multi-rotors which will be carrying payloads, a wider bell with more torque is preferred. This gives the multi-rotor increased lift capacity. The motors are rated by their size in millimeters and by a kv rating system. Kv stands for revolutions per volt and in a perfect world if you had a 1000 kv motor you would get 1000 revolutions per volt. The kv number to torque ratio has an inverse relationship, in that the larger a motor is and the more torque it has, the lower its kv rating will be. Conversely, smaller motors with less torque but more revolutions per minute will have higher kv ratings. A good starting point for a quad copter with the capacity to carry a medium sized camera would be a motor with a kv rating between 700 and 1000.
Motors are also rated by amperage draw. This is more of a linear relationship in that the greater the torque and thrust and the larger the propeller the more amps the motor will draw under load. Always make sure that the ESC units you use are rated to sustain the amount of amps your motor and propeller combination will draw. Failure to remember this will result in a crashed multi-rotor.
Flight Control Units
Flight Control Units go by many names and vary widely in pricing and features. The FCU is what controls all of the attached ESC units, which in turn control the motor’s revolutions per minute, thus making controlled level flight possible.
But that is only scratching the surface of what they can do. Their prices are based on their features and you can get good FCU for prices starting around $15 and ranging up into the thousands. Which one is right for you depends entirely on what type of mission you are trying to accomplish with your multi-rotor. Even the least expensive FCU will feature integrated 3 axis gyros. The FCU software interpretation of the input data from the gyros is what allows the board to have a type of spatial awareness. It will in turn use this information to control power output to each motor. It does this continuously in a loop allowing for stable flight of a multi-rotor aircraft.
Mid range Flight Control Units will usually incorporate additional sensors. These usually include a GPS unit with antenna and a barometer. The GPS unit adds geospatial awareness to the FCU. In this way the unit can incorporate its GPS position, speed, heading, and altitude data. This increases the accuracy and stability of the multi-rotor. One of the down sides with GPS data is the fact that it is very inaccurate, especially in regards to altitude. GPS has a tendency to wander both horizontally and vertically. Hence the introduction of the barometer. When the FCU is armed before take off the barometer on the board will take a reading. That reading becomes 0 ft. altitude. As the multi-rotor lifts off, the values sampled by the barometer change and the FCU can use this data in addition to the GPS data to make very accurate altitude predictions. On average the altitude prediction will be within the range of plus or minus 1 ft.
Higher end Flight Control Units add a variety of additional features. These include digital compasses and software suites for modifying settings and planning way point based missions. Missions which the multi-rotor can fly from take off to landing completely autonomously.
The ability to stream commands and real time telemetry data between your laptop to your multi-rotor is also possible. There are many different possibilities and many different controllers for many tasks. Figure out what you want to do and shop around.
Some popular and cost effective boards are:
It is important to read as much about these boards as you can find. It will make finding the right board for your application much easier. Probably the best repository for all things remote controlled is RcGroups. The RC Groups forum contains a wealth of knowledge and helpful people. You will be able to find highly detailed threads on every one of the Flight Control Boards and much more.
For the purposes of this project I chose to use the ArduPilot Mega 2.5 from DIYdrones. It has a robust feature set, a large active community and the software is created and maintained by that community. Because of this world wide cooperation the APM 2.5 performs on a level that matches that of controllers which cost ten times its price.
The Transmitter and Receiver
Arguably the most important part of the system is the transmitter and receiver. This system is how your physical inputs become motion with the multi-rotor. The better the system the further your multi-rotor can go. For this project and all other RC related projects I use a modified version of the Turnigy 9x which is an inexpensive, fully featured RC controller which runs on the 2.4 GHz radio band.
The modifications I have done include adding the custom thumb stick ends, a back light for the LCD screen, and aftermarket transmitter module, a high gain panel antenna, and a LiFe battery. Below you can see the upgraded module.
This allows me to use custom antennas and much more reliable receivers which feature telemetry data. I also use a custom firmware (operating system) on the transmitter called Er9x. It expands the transmitter model memory and makes complex configurations much easier. Many people who have been in the RC world for years swear by this setup. It is very cost effective as well; my radio with all the modifications cost around $100 dollars. Other radios with the same feature set can cost $400 dollars and up into the thousands. Here is an example of a receiver for this system.
The majority of transmitters and receivers run on two different frequencies – 2.4 GHz and 433 MHz. Both frequencies have their advantages and disadvantages and for the most part 2.4 GHz is used for short range and 433 MHz for long range. 2.4 GHz users can expect ranges with stock transmitters of about 3 to 5 kilometers. People who use the 433 MHz band with stock transmitters can achieve distances of up to 20 kilometers. The vast disparity in range comes down to wave lengths. The higher the frequency the shorter the wave length the more likely it is the signal will bounce off of an object rather than permeate it. The lower the frequency the longer the wavelength, the farther it will travel at the same transmission power and the more likely it will be to penetrate solid objects. The systems are also dramatically different in price, with the long range systems costing 3 to 10 times more. For the purposes of this project I will be using the 2.4 GHz system because I wont usually be flying beyond the line of sight and it is more cost effective.